Checking for blocked ports from a remote computer when TELNET is not available (4229067)

• Increasingly customers are reluctant to install Telnet on their servers for security reasons.

Using a browser

Google Chrome

Port blocked “refused to connect” or is not listening

Port is open (but is rejected for various reasons because it is not a web browser port)

Internet Explorer

Port blocked or not listening

Port open but rejected

Mozilla Firefox

Port blocked or not listening

Port Open

In LINUX From the command line

Using Netcat with CentOS

Output a new blank command line or message “connection reset by peer” if the port is open; Output “Connection refused” if the port is closed.

Port is open

[root@foglight /]# nc 192.168.1.48 8080 < /dev/null
[root@foglight /]#

[root@foglight /]# nc 192.168.1.48 135 < /dev/null
[root@foglight /]#

[root@foglight /]# nc 192.168.1.41 1521 < /dev/null
[root@foglight /]#

“Connection refused” if the port is closed.

[root@foglight /]# nc 192.168.1.48 8081 < /dev/null
Ncat: Connection refused.

Using Netcat with Oracle Linux or RHEL

Port is open

Using Curl (with CentOS)

Construct an HTTP URI from the hostname and port as the first argument to curl. If curl can connect, it will report a protocol mismatch and exit (if the listener isn't a web service). If curlcannot connect, it will time out.

This is a blocked port either closed or blocked by a firewall

[root@foglight /]# curl 192.168.1.48:8081

curl: (7) Failed connect to 192.168.1.48:8081; Connection refused

For example a PI repository is running on this remote server

[root@foglight /]# curl 192.168.1.48:5029

curl: (56) Recv failure: Connection reset by peer

This is an open FMS webpage, there is no error and a new command line entry is displayed

[root@foglight /]# curl 192.168.1.48:8080

[root@foglight /]#

This is open Windows RPC port, this will establish a connection. Use Ctrl-C to end the connection

[root@foglight /]# curl 192.168.1.48:135

^C

[root@foglight /]#

This is an open SQL Server port

[root@foglight /]# curl 192.168.1.50:1433

curl: (52) Empty reply from server

This is an open Oracle port

[root@foglight /]# curl 192.168.1.41:1521

curl: (52) Empty reply from server

Using Echo (this must be done on the remote host)

This is an open local port

[root@foglight /]# echo > /dev/tcp/localhost/80 && echo Port Open || echo Port closed

-bash: connect: Connection refused
-bash: /dev/tcp/localhost/80: Connection refused
Port closed